Materials for printing electrical circuits on electrical conductor substrates are disclosed in U.S. Pat. Nos. 5,882,722, 6,036,889, 6,143,356 and 6,379,745, the entire disclosures of which are expressly incorporated herein by reference, and are known as PARMOD® materials. PARMOD® materials have been developed for printing conductive circuits on polymer or paper substrates such as those used for printed wiring boards, flexible circuits and RFID antennae. Using PARMOD® materials and a simple print-and-heat process for “chemical welding” of pure metals, electrical conductors made of a single-phase continuous well-bonded metal trace are produced, rather than conductors made of individual particles that may be in adventitious contact with each other, as are found in polymer thick film containing materials. PARMOD® materials provide a desirable alternative to the conventional polymer thick film compositions that are cured at high temperatures onto ceramic or glass based substrates. PARMOD® materials are cured at temperatures which polymer and paper based substrates can withstand, and provide electrical conductivity comparable to the pure metal and at least a factor of five greater than most known polymer thick films.
PARMOD® compositions have been printed on polyimide films coated with various adhesive layers and thermally cured to create flexible printed circuits. Suitable substrates are: KAPTON® type FN with a FEP TEFLON® coating; KAPTON types KJ and LJ with low melting polyimide coatings; and polyimide substrates with a polyamic acid coating. PARMOD® compositions have been printed directly on certain grades of FR-4 epoxy-glass laminates and thermally cured to produce well-bonded rigid printed circuits.
However, a significant problem in the manufacture of PARMOD® products is that thermal treatment of PARMOD® materials can damage low-temperature substrates such as polymer or paper, and cause mechanical and dimensional instability to the printed circuits. For example, the cure temperature for “chemically welding” PARMOD® silver into circuit traces is 200° C. This curing temperature limits the choice of substrates to those with high thermal resistance, such as polyimides and epoxies. Many polymer and paper substrates cannot be processed at these temperatures.
Although low temperature sintering of metal toners has been observed on certain substrates, see, e.g., WO 01/45935, specific temperature lowering additives to promote low temperature sintering have not heretofor been defined or shown to be advantageous in the manufacture of conductive ink compositions.
Thus, there is a need for PARMOD® methods and compositions such as inks which have a minimum cure temperature that is compatible with substrates of commercial interest and which still retain the highly conductive properties of the PARMOD® materials.